I fitted heated washer jets on mine in 1986, but what I did was wrap about 6 coils of copper brake pipe around a 3/4" copper pipe, all soldered together, then plumbed the big pipe into the cooling system and passed the washer fluid through the brake pipe...

My heater was air blending, so that I could get rid of that valve on the cylinder head, and that meant the washer jets weren't affected by the heat in the car. Although I suppose that probably wouldn't have mattered.
At one time I noticed a similar system for sale in a local car accessory shop. I did mine before that, though!